Reciprocating-piston internal combustion engines, which will in this context and hereinafter also be referred to in shortened form merely as internal combustion engines, have one or more cylinders in which a reciprocating piston is arranged. To illustrate the principle of a reciprocating-piston internal combustion engine, reference will be made below to FIG. 1, which illustrates by way of example a cylinder of an internal combustion engine, which is possibly also a multi-cylinder internal combustion engine, together with the most important functional units.
The respective reciprocating piston 6 is arranged in linearly movable fashion in the respective cylinder 2 and, together with the cylinder 2, encloses a combustion chamber 3. The respective reciprocating piston 6 is connected by means of a so-called connecting rod 7 to a respective crankpin 8 of a crankshaft 9, wherein the crankpin 8 is arranged eccentrically with respect to the crankshaft axis of rotation 9a. As a result of the combustion of a fuel-air mixture in the combustion chamber 3, the reciprocating piston 6 is driven linearly “downward”.
The translational stroke movement of the reciprocating piston 6 is transmitted by means of the connecting rod 7 and crankpin 8 to the crankshaft 9 and is converted into a rotational movement of the crankshaft 9, which causes the reciprocating piston 6, owing to its inertia, after it passes through a bottom dead center in the cylinder 2, to be moved “upward” again in the opposite direction as far as a top dead center. To permit continuous operation of the internal combustion engine 1, during a so-called working cycle of a cylinder 2, it is necessary firstly for the combustion chamber 3 to be filled with the fuel-air mixture, for the fuel-air mixture to be compressed in the combustion chamber 3 and to then be ignited (by means of an ignition plug in the case of a gasoline internal combustion engine and by ultra-ignition in the case of a diesel internal combustion engine) and burned in order to drive the reciprocating piston 6, and finally for the exhaust gas that remains after the combustion to be discharged from the combustion chamber 3. Continuous repetition of this sequence results in continuous operation of the internal combustion engine 1, with work being output in a manner proportional to the combustion energy.
Depending on the engine concept, a working cycle of the cylinder is divided into two strokes distributed over one crankshaft rotation (360°) (two-stroke engine) or into four strokes distributed over two crankshaft rotations (720°) (four-stroke engine). To date, the four-stroke engine has become established as a drive for motor vehicles. In an intake stroke, with a downward movement of the reciprocating piston 6, fuel-air mixture 21 (in the case of intake pipe injection by means of injection valve 5a, illustrated as an alternative in FIG. 1 by means of dashed lines) or else only fresh air (in the case of fuel direct injection by means of injection valve 5) is introduced from the intake tract 20 into the combustion chamber 3. During the following compression stroke, with an upward movement of the reciprocating piston 6, the fuel-air mixture or the fresh air is compressed in the combustion chamber 3, and if appropriate fuel is separately injected by means of an injection valve 5. During the following working stroke, the fuel-air mixture, for example in the case of the gasoline internal combustion engine, is ignited by means of an ignition plug 4, burns and expands, outputting work, with a downward movement of the reciprocating piston 6. Finally, in an exhaust stroke, with another upward movement of the reciprocating piston 6, the remaining exhaust gas 31 is discharged out of the combustion chamber 3 into the exhaust-gas tract 30.
The delimitation of the combustion chamber 3 with respect to the intake tract 20 or exhaust-gas tract 30 of the internal combustion engine 1 is realized generally, and in particular in the example taken as a basis here, by means of inlet valves 22 and outlet valves 32. In FIG. 1, said valves are actuated by means of at least one camshaft. The example shown has an inlet camshaft 23 for actuating the inlet valves 22 and has an outlet camshaft 33 for actuating the outlet valves 32. There are normally yet further mechanical components (not illustrated here) for force transmission provided between the valves and the respective camshaft, which components may also include a valve play compensation means (e.g. bucket tappet, rocker lever, finger-type rocker, tappet rod, hydraulic tappet etc.).
The inlet camshaft 23 and the outlet camshaft 33 are driven by means of the internal combustion engine 1 itself. For this purpose, the inlet camshaft 23 and the outlet camshaft 33, in each case by means of suitable inlet camshaft control adapters 24 and outlet camshaft control adapters 34, such as for example toothed gears, sprockets or belt pulleys, and with the aid of a control mechanism 40, which has for example a toothed gear mechanism, a control chain or a toothed control belt, are coupled, in a predefined position with respect to one another and with respect to the crankshaft 9 by means of a corresponding crankshaft control adapter 10, which is correspondingly formed as a toothed gear, sprocket or belt pulley, to the crankshaft 9. By means of this connection, the rotational position of the inlet camshaft 23 and of the outlet camshaft 33 in relation to the rotational position of the crankshaft 9 is, in principle, defined. By way of example, FIG. 1 illustrates the coupling between inlet camshaft 23 and the outlet camshaft 33 and the crankshaft 9 by means of belt pulleys and a toothed control belt.
The rotational angle covered by the crankshaft during one working cycle will hereinafter be referred to as working phase or simply as phase. A rotational angle covered by the crankshaft within one working phase is accordingly referred to as phase angle. The respectively current crankshaft phase angle of the crankshaft 9 can be detected continuously by means of a position encoder 43 connected to the crankshaft 9, or to the crankshaft control adapter 10, and an associated crankshaft position sensor 41. Here, the position encoder 43 may be formed for example as a toothed gear with a multiplicity of teeth arranged so as to be distributed equidistantly over the circumference, wherein the number of individual teeth determines the resolution of the crankshaft phase angle signal.
It is likewise additionally possible, if appropriate, for the present phase angles of the inlet camshaft 23 and of the outlet camshaft 33 to be detected continuously by means of corresponding position encoders 43 and associated camshaft position sensors 42. Since, owing to the predefined mechanical coupling, the respective crankpin 8, and with the latter the reciprocating piston 6, the inlet camshaft 23, and with the latter the respective inlet valve 22, and the outlet camshaft 33, and with the latter the respective outlet valve 32, move in a predefined relationship with respect to one another and in a manner dependent on the crankshaft rotation, said functional components run through the respective working phase synchronously with respect to the crankshaft. The respective rotational positions and stroke positions of reciprocating piston 6, inlet valves 22 and outlet valves 32 can thus, taking into consideration the respective transmission ratios, be set in relation to the crankshaft phase angle of the crankshaft 9 predefined by the crankshaft position sensor 41. In an ideal internal combustion engine, it is thus possible for every particular crankshaft phase angle to be assigned a particular crankpin angle, a particular piston stroke, a particular inlet camshaft angle and thus a particular inlet valve stroke and also a particular outlet camshaft angle and thus a particular outlet camshaft stroke. That is to say, all of the stated components are, or move, in phase with the rotating crankshaft 9.
In modern internal combustion engines 1, there may be additional positioning elements provided within the mechanical coupling path between crankshaft 9 and inlet camshaft 23 and the outlet camshaft 33, for example in a manner integrated into the inlet camshaft adapter 24 and the outlet camshaft adapter 34, which positioning elements effect a desired controllable phase shift between the crankshaft 9 and inlet camshaft 23 and the outlet camshaft 33. These are known as so-called phase adjusters in so-called variable valve drives. For optimum operation of the internal combustion engine (with regard to emissions, consumption, power, running smoothness, etc.), the fresh-gas charge drawn in during the intake stroke should be known as accurately as possible, and the metered fuel quantity should be coordinated therewith as exactly as possible, in order for example to be able to ensure operation with lambda (λ)=1, that is to say with the minimum quantity of oxygen required for the complete combustion of the fuel to be metered.
The drawn-in fresh-gas charge is dependent on various factors, such as for example the structural details of the internal combustion engine, the present operating point and the present settings of different positioning devices, such as for example of a throttle flap. A typical method for determining the fresh-gas charge is to measure a so-called reference internal combustion engine in all occurring operating states (engine speed, load, actuation of all actuators, different valve strokes, actuation of flaps, actuation of the phase adjusters for inlet and outlet valve, exhaust-gas turbocharger, compressor etc.), and to store said measurement values (or derivatives thereof or model-based approaches which replicate the behavior) in corresponding characteristic maps in the engine control unit of a corresponding series-production internal combustion engine. All structurally identical, series-production internal combustion engines of the same type series are operated with this reference dataset that is generated. As a first approximation, the fresh-gas charge can thus be assumed as being known.
The associated fuel quantity to be metered, in particular to be injected, is then calculated in accordance with the predefined air/fuel ratio (A/F ratio) of the respective fuel, which is dependent on the fuel type and the fuel quality or the fuel composition. Differences thus arise depending on the fuel used. Accordingly, for example in the case of mixed fuels composed of premium gasoline and ethanol, the following air/fuel ratios are present:
100 vol % premium gasoline 0 vol % ethanol (E0) => 14.575 vol % premium gasoline25 vol % ethanol (E25) => 13.150 vol % premium gasoline50 vol % ethanol (E50) => 11.825 vol % premium gasoline75 vol % ethanol (E75) => 10.4 0 vol % premium gasoline100 vol % ethanol (E100) => 9.0 
The ethanol fraction is stated in the designation of the mixed fuel in each case in percent by volume, that is to say the mixed fuel E25 is accordingly composed of 75 vol % premium gasoline and 25 vol % ethanol.
For optimum operation of the internal combustion engine with regard to consumption, running smoothness, and emissions, it is thus also necessary for the composition of the fuel used in the respectively present operating mode to be known as accurately as possible, because otherwise erroneously metered fuel quantities may arise. Furthermore, different fuels or fuel compositions may have different characteristics, such as for example different knock resistances. For optimization of the operation, this may necessitate further adaptations, for example of the injection time or of the ignition time.
Since fuels are possibly not always available in the same quality or composition, differences may arise here from tank filling to tank filling, which differences adversely affect the operation of the internal combustion engine. For this reason, various methods and devices are already known from the prior art which have the aim of determining the fuel composition or the fuel quality.
For example, document DE 10 2009 031 159 B3 discloses a method and a device for determining a fuel quality, in particular a mixed composition of a fuel. The method is based on the determination of an electrical parameter of the fuel by means of an electric motor which has a stator and a rotor, wherein, in a gap between rotor and stator, an electrical parameter of the fuel situated therein is determined, which electrical parameter is a measure of the fuel quality.
Also known from document DE 10 2009 017 207 B4, for example, is a method for detecting the fuel quality, in which method a fuel quality value is calculated by means of a fuel quality calculation module on the basis of a torque of the engine and a change in engine speed measured over a first time period.
Furthermore, document DE 10 2011 077 404 B4 also discloses a method for determining the fuel type, which method is based on a highly precise feed of a particular differential fuel delivery quantity into a high-pressure fuel accumulator. From the corresponding pressure increase curve, a measurement value curve is determined, which is compared with comparison value curves, stored in an associated control device, for different fuel qualities. In the case of sufficient correlation of the measurement value curve with a comparison value curve, the associated fuel quality is determined.
The known methods commonly require additional sensors or, owing to environmental influences which are difficult to detect, are complicated to implement and give unsatisfactory results.